The use of the one device cell, invented by Dennard in 1967 (see generally, U.S. Pat. No. 3, 387,286, issued to R. H. Dennard on Jun. 4, 1968, entitled "Field Effect Transistor memory"), revolutionized the computer industry, by significantly reducing the complexity of semiconductor memory. This enabled the cost, of what was then a scarce commodity, to be drastically reduced.
Today, dynamic random access memories (DRAMs) are a mainstay in the semiconductor industry. DRAMs are data storage devices that store data as charge on a storage capacitor. A DRAM typically includes an array of memory cells. Each memory cell includes a storage capacitor and an access transistor for transferring charge to and from the storage capacitor. Each memory cell is addressed by a word line and accessed by a bit line. The word line controls the access transistor such that the access transistor controllably couples and decouples the storage capacitor to and from the bit line for writing and reading data to and from the memory cell. Current DRAM technology generally requires a refreshing of the charge stored on the storage capacitor where the charge must be refreshed every so many milliseconds.
Over the course of time what was a very simple device (a planer capacitor and one transistor) has, because of even shrinking dimensions, become a very complex structure to build. Whether it is the trench capacitor, favored by IBM, or the stacked capacitor, used by much of the rest of the industry, the complexity and difficulty has increased with each generation. Many different proposals have been proposed to supplant this device, but each has fallen short because of either the speed of the write or erase cycle being prohibitively long or the voltage required to accomplish the process too high. One example of the attempt to supplant the traditional DRAM cell is the so-called electrically erasable and programmable read only memory (EEPROM), or more common today, flash memory.
Electrically erasable and programmable read only memories (EEPROMs) provide nonvolatile data storage. EEPROM memory cells typically use field-effect transistors (FETs) having an electrically isolated (floating) gate that affects conduction between source and drain regions of the FET. A gate dielectric is interposed between the floating gate and an underlying channel region between source and drain regions. A control gate is provided adjacent to the floating gate, separated therefrom by an intergate dielectric.
In such memory cells, data is represented by charge stored on the polysilicon floating gates. The charge is placed on the floating gate during a write operation using a technique such as hot electron injection or Fowler-Nordheim (FN) tunneling. Fowler-Nordheim tunneling is typically used to remove charge from the polysilicon floating gate during an erase operation. A flash EEPROM cell has the potential to be smaller and simpler than a DRAM memory cell. One of the limitations to shrinking a flash EEPROM memory cell has been the requirement for a silicon dioxide gate insulator thickness of approximately 10 nm, or 100 .ANG., between the floating polysilicon gate and the silicon substrate forming the channel of a flash field effect transistor. This gate thickness is required to prevent excess charge leakage from the floating gate that would reduce data retention time (targeted at approximately 10 years)
Current n-channel flash memories utilize a floating polysilicon gate over a silicon dioxide gate insulator of thickness of the order 100 .ANG. or 10 nm in a field effect transistor. (See generally, B. Dipert et al., IEEE Spectrum, pp. 48-52 (Oct. 1993). This results in a very high barrier energy of around 3.2 eV for electrons between the silicon substrate and gate insulator and between the floating polysilicon gate and silicon oxide gate insulator. This combination of barrier height and oxide thickness results in extremely long retention times even at 250 degrees Celsius. (See generally, C. Papadas et al., IEEE Trans. on Electron Devices, 42, 678-681 (1995)). The simple idea would be that retention times are determined by thermal emission over a 3.2 electron volt (eV) energy barrier, however, these would be extremely long so the current model is that retention is limited by F-N tunneling off of the charged gate. This produces a lower "apparent" activation energy of 1.5 eV which is more likely to be observed. Since the retention time is determined either by thermal excitation of electrons over the barrier or the thermally assisted F-N tunneling of electrons through the oxide, retention times are even longer at room temperature and/or operating temperatures and these memories are for all intensive purposes non-volatile and are also known as non-volatile random access memories (NVRAMs). This combination of barrier height and oxide thickness tunnel oxide thickness is not an optimum value in terms of transfer of electrons back and forth between the substrate and floating gate and results in long erase times in flash memories, typically of the order of milliseconds. To compensate for this, a parallel erase operation is performed on a large number of memory cells to effectively reduce the erase time, whence the name "flash" or "flash EEPROM" originated since this effective erase time is much shorter than the erase time in EEPROMs.
Certain approaches to reduce the long erase times in conventional n-channel flash memory cells have been described. These approaches describe the use of reduced barriers between the substrate and polysilicon floating gate and gate insulator by using a different gate insulator rather than silicon dioxide; or using a material other than polysilicon for the floating gate to reduce the barrier between the floating gate and gate insulator. Other structures and methods, disclosed previously to increase the tunneling current and reduce the erase time, have included the use of roughened silicon surfaces under the tunnel oxide to locally increase electric fields.
However, there is yet a need in the art to develop a generalized method for modifying n-channel floating gate transistors such that the same can usefully be implemented in deep sub-micron CMOS technology devices, i.e. which can replace DRAM cells in CMOS technology devices. That is, it is desirable to develop n-channel floating gate transistors which are more responsive, providing faster write and erase times while still maintaining a long life cycle. It is desirable that such n-channel floating gate transistors scale with the shrinking design rules and lower operating voltages in CMOS technology. And, it is further desired that any required refreshing, as ordinarily required in DRAM cells to restore a charge to the memory cell, have an increased refreshing time interval of greater than millisecond intervals.